The present invention relates to a coplanar transmission line, and more particularly to a coplanar transmission line used in a high frequency circuit, wherein the coplanar transmission line is improved in reduction of a transmission loss and also in keeping a sufficient chemical strength.
The coplanar transmission line has been used in the high frequency circuit. The coplanar transmission line has the following structure. A signal conductive layer is selectively formed on a surface of a dielectric substrate, wherein the signal conductive layer allows a signal transmission. Ground conductive layers are selectively formed on the top surface of the dielectric substrate, so that the ground conductive layers extend along opposite sides of the signal conductive layer. A bottom ground conductive layer is formed on a bottom surface of the dielectric substrate for mounting the substrate to an external case, wherein the bottom ground conductive may be made of a metal such as gold or tin. The ground conductive layers on the top surface of the dielectric substrate and the bottom ground conductive layer on the bottom surface of the dielectric substrate do form a parallel-plate structure which allows a parallel-plate mode signal as a leakage signal to be propagated through the dielectric substrate and in a direction perpendicular to a signal transmission direction in which the signal is transmitted in the signal conductive layer or the co-planer transmission line. As a frequency of the signal transmitted or propagated through the coplanar transmission line is high, a signal waveguide mode is coupled to the parallel-plate mode, whereby the signal transmission loss is increased. Further, even if a distance between adjacent two of the coplanar transmission lines is large, the signal waveguide mode is made coupled to the parallel-plate mode whereby coupling of the signal waveguide mode between the adjacent two of the coplanar transmission lines appears through the parallel-plate mode. Namely, it is difficult for the conventional structure of the coplanar transmission line to prevent the signal transmission loss due to the coupling in the parallel-plate mode between the adjacent two of the coplanar transmission lines.
In order to have solved the above problem with the signal transmission loss due to the coupling between the adjacent two of the coplanar transmission lines, another conventional structure of the coplanar transmission line has been proposed, which is disclosed in Nirod K. Das xe2x80x9cIEEE Transactions on Microwave Theory and Techniquesxe2x80x9d, Vol. 44, pp. 169-181, 1996. This conventional structure of the coplanar transmission line has a plurality of metal via which provide connections between the ground conductive layers on the top surface of the dielectric substrate and the bottom ground conductive layer on the bottom surface of the dialectic substrate. FIG. 1 is a fragmentary cross sectional elevation view illustrative of the conventional structure of the coplanar transmission line, wherein the plurality of metal via is provided for providing connections between the ground conductive layers on the top surface of the dielectric substrate and the bottom ground conductive layer on the bottom surface of the dialectic substrate.
A signal conductive layer 22 is selectively formed on a top surface of a dielectric substrate 21 which is made of a ceramic. Two ground conductive layers 23 are selectively formed on the top surface of the dielectric substrate 21, so that the ground conductive layers 23 extend in opposite sides of the signal conductive layer 22, provided that the ground conductive layers 23 are distanced from opposite side edges of the signal conductive layer 22. A bottom ground conductive layer 24 is formed entirely on a bottom surface of the dielectric substrate 21. Two metal vias 25 are formed in the dielectric substrate 21 so that the two ground conductive layers 23 are connected through the two metal vias 25 to the bottom ground conductive layer 24. The metal vias 25 may prevent the signal or power leakage in the parallel-plate mode.
Still another conventional structure of the coplanar transmission line is disclosed in M. Hotta Asia Pacific Microwave Conference 1998 Proceedings, pp. 409-412. FIG. 2 is a fragmentary cross sectional elevation view illustrative of the still other conventional structure of the coplanar transmission line, wherein a groove is formed. A groove is formed in the dielectric substrate, so that the groove extends under the signal conductive layer and over the bottom ground conductive layer. A signal conductive layer 32 is selectively formed on a top surface of a dielectric substrate 31. Two ground conductive layers 33 are selectively formed on the top surface of the dielectric substrate 31, so that the ground conductive layers 33 extend in opposite sides of the signal conductive layer 32, provided that the ground conductive layers 33 are distanced from opposite side edges of the signal conductive layer 32. A bottom ground conductive layer 34 is formed entirely on a bottom surface of the dielectric substrate 31. A groove 36 is formed in the dielectric substrate 31, so that a bottom of the groove 36 is positioned directly over the bottom ground conductive layer 34, whilst a top of the groove 36 is positioned indirectly under the signal conductive layer 32 and also under inside portions of the ground conductive layers 33, provided that the top of the groove 36 is separated by a part of the dielectric substrate 31 from the signal conductive layer 32 and the ground conductive layers 33. The existence of the groove 36 makes a reduction in thickness of the part of the dielectric substrate 31 under the signal conductive layer 32, whereby the signal or power leakage in the parallel-plate mode is cut off.
Yet another conventional structure of the coplanar transmission line is disclosed in Japanese Laid-open Patent Publication No. 9-23106. A signal conductive layer is selectively formed on a top surface of a dielectric substrate. Two ground conductive layers are selectively formed on the top surface of the dielectric substrate, so that the ground conductive layers extend in opposite sides of the signal conductive layer, provided that the ground conductive layers are distanced from opposite side edges of the signal conductive layer. A bottom ground conductive layer is formed entirely on a bottom surface of the dielectric substrate. Two metal vias are formed in the dielectric substrate so that the two ground conductive layers are connected through the two metal vias to the bottom ground conductive layer. The metal vias may prevent the signal or power leakage in the parallel-plate mode. Additionally, four recessed portions are formed in opposite edges of the dielectric substrate, wherein the opposite edges are distanced in the same direction as the signal transmission or signal propagation through the signal conductive layer. At the opposite edges of the dielectric substrate, a discontinuity in the transmission or propagation mode is likely to appear. The recessed portions suppress generation of the higher-order propagation mode.
An additional conventional structure of the coplanar transmission line is disclosed in Japanese Laid-open Patent Publication No. 9-23107. A signal conductive layer is selectively formed on a top surface of a dielectric substrate. Two ground conductive layers are selectively formed on the top surface of the dielectric substrate, so that the ground conductive layers extend in opposite sides of the signal conductive layer, provided that the ground conductive layers are distanced from opposite side edges of the signal conductive layer. A bottom ground conductive layer is formed entirely on a bottom surface of the dielectric substrate. Two metal vias are formed in the dielectric substrate so that the two ground conductive layers are connected through the two metal vias to the bottom ground conductive layer. The metal vias may prevent the signal or power leakage in the parallel-plate mode. Additionally, a shielding ground conductive cover is provided which covers the inside portions of the two ground conductive layers and the signal conductive layer, wherein the shielding ground conductive cover is in contact with the two ground conductive layers. Furthermore, a connecting side of the dielectric substrate which is to be connected with a co-axial connector is modified in shape of the two ground conductive layers on the top surface of the dielectric substrate. At the connecting side, the two ground conductive layers are made distanced from the signal conductive layer. Namely, the distance of the two ground conductive layers from the signal conductive layer is increased at the connecting side for improvement in impedance-matching at the connecting side which is to be connected with the co-axial connector.
The above described conventional structures of the coplanar transmission lines have individual disadvantages as follows.
The conventional structure of the coplanar transmission line illustrated in FIG. 1 has a number of the metal via for suppressing the signal or power leakage to the parallel-plate mode, whereby this structure causes a problem that the dielectric substrate is insufficient in mechanical strength and thermal-stress stability.
If the conventional structure of the coplanar transmission line having a plurality of via is applied to a sealed package, a problem is caused in a possible leakage of air from the bottom surface of the dielectric substrate. FIG. 3 is a schematic perspective view illustrative of a sealed package for the conventional structure of the coplanar transmission line having a plurality of via. A first multi-layer dielectric substrate 21a has a top surface on which a signal conductive layer 22a and two ground conductive layers 23a are formed, so that the two ground conductive layers 23a extend in opposite sides of the signal conductive layer 22a and also the two ground conductive layers 23a are separated from the signal conductive layer 22a. A plurality of via 25a is formed which penetrates through the first multi-layer dielectric substrate 21a. The plurality of via 25a are aligned along inside edges of the two ground conductive layers 23a. A semiconductor device 27 is mounted on a center portion of the first multi-layer dielectric substrate 21a, so that a bottom surface of the semiconductor device 27 is in contact with the signal conductive layer 22a and the inside portions of the two ground conductive layers 23a. A second multi-layer dielectric substrate 21b is provided on the top surface of the first multi-layer dielectric substrate 21a. The second multi-layer dielectric substrate 21b has a large square-shaped opening in which the semiconductor device 27 is accommodated. A size of the square-shaped opening is much larger than the semiconductor device 27, so that the inside edges of the second multi-layer dielectric substrate 21b is separated from the semiconductor device 27. The second multi-layer dielectric substrate 21b is thicker than the semiconductor device 27. A top cover 26 is provided on the top surface of the second multi-layer dielectric substrate 21b, wherein the top cover 26 has substantially the same size as the second multi-layer dielectric substrate 21b, so that the semiconductor device 27 is accommodated within an internal space defined by the top cover 26, the second multilayer dielectric substrate 21b and the first multi-layer dielectric substrate 21a. This packaging structure allows a possible leakage of air from the bottom surface of the first multi-layer dielectric substrate 21a. 
The conventional structure of the coplanar transmission line illustrated in FIG. 2 has the groove so provided that the thickness of the part of the dielectric substrate under the signal conductive layer and over the groove is reduced, whereby this structure causes a problem that the dielectric substrate is insufficient in mechanical strength.
The conventional structure of the coplanar transmission line disclosed in Japanese Laid-open Patent Publication No. 9-23106 has a number of the metal via for suppressing the signal or power leakage to the parallel-plate mode, whereby this structure causes a problem that the dielectric substrate is insufficient in mechanical strength and thermal-stress stability.
The conventional structure of the coplanar transmission line disclosed in Japanese Laid-open Patent Publication No. 9-23107 also has a number of the metal via for suppressing the signal or power leakage to the parallel-plate mode, whereby this structure causes a problem that the dielectric substrate is insufficient in mechanical strength and thermal-stress stability.
In the above circumstances, it had been required to develop a novel coplanar transmission line structure free from the above problems.
Accordingly, it is an object of the present invention to provide a novel coplanar transmission line structure free from the above problems.
It is a further object of the present invention to provide a novel coplanar transmission line structure which reduces a transmission loss.
It is a still further object of the present invention to provide a novel coplanar transmission line structure which allows a dielectric substrate to have a high mechanical strength.
It is yet a further object of the present invention to provide a novel coplanar transmission line structure which allows a dielectric substrate to have a high thermal stress stability.
The first present invention provides a transmission line structure comprising: a dielectric substrate having first and second surfaces; a signal conductive layer selectively provided on the first surface of the dialectic substrate for signal transmission; at least a first non-signal conductive layer being selectively provided on the first surface of the dialectic substrate, and the at least first non-signal conductive layer being separated from the signal conductive layer and a second non-signal conductive layer being provided on the second surface of the dialectic substrate, wherein the dielectric substrate has at least a conductive region extending in contact with only one of the at least first non-signal conductive layer and the second non-signal conductive layer so that the at least conductive region is separated by the dielectric substrate from remaining one of the first non-signal conductive layers and the second non-signal conductive layer in view of a vertical direction to the first and second surfaces of the dielectric substrate.
The second present invention provides a transmission line structure comprising: a dielectric substrate having first and second surfaces; a signal conductive layer selectively provided on the first surface of the dialectic substrate for signal transmission; at least a first non-signal conductive layer being selectively provided on the first surface of the dialectic substrate, and the at least first non-signal conductive layer being separated from the signal conductive layer; and a second non-signal conductive layer being provided on the second surface of the dialectic substrate, wherein the dielectric substrate has at least a low dielectric constant region which is lower in dielectric constant than the dielectric substrate and which does at least extend under a part of the first non-signal conductive layer near the signal conductive layer, whereby there is reduced an effective dielectric constant of the dielectric substrate receiving a field generated from the signal on transmission through the signal conductive line, so that the transmission loss is reduced.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.